1. Field of the Invention
This invention relates to a new noninvasive method for calculating Actual Stroke Volume and Cardiac Output of a human heart using computerized algorithms, and to a continuous noninvasive hemodynamic analyzer which computes a complete real-time Hemodynamic Profile in rest and exercise.
2. Description of Prior Art
The principal role of Cardiac Output in the Hemodynamic Hierarchy has resulted in a long felt need for an accurate measurement of blood flow. The fact is that if Cardiac Output is known, a whole sequence of cardiovascular parameters can be calculated with the values of the easily obtainable Blood Pressure Data. A noninvasive, continuous/frequently reapplicable, patient-, and user-friendly, inexpensive method could revolutionize modern health care by treating patients better at less expense.
There are about eight useful methods at the present time for measuring Cardiac Output. The first three are the Fick Method, the Dye-Indicator Dilution Method, and the Thermodilution Method. Three other methods allow visualization of the heart chambers; these include cine-angiography, gated-pool radionuclides, and echocardiography. The two most practical noninvasive methods at the present time are Doppler Ultrasound and Electrical Bioimpedance.
Although all these methods have some merit, there is no absolutely accurate method which could provide a standard to measure Cardiac Output. It is generally accepted in hemodynamic literature that all methods have an inherent inaccuracy of .+-.10% and their absolute accuracy at best is .+-.15-20%. Therefore, if any two methods are compared to each other, at least 75% of the points should fall within a .+-.20% confidence band to establish good correlation.
A problem in the field of Cardiac Output-Measurement is that none of the methods fulfills all ideal requirements. In the following an attempt is made to summarize the ideal requirements on a 100 point scale. The different categories are arbitarily assigned the same weight for the sake of comparability.
1. Maximum inherent inaccuracy .+-.10% and absolute accuracy at best .+-.20% (10 points).
2. General availability for any kind of patient, considering:
a) no size and/or weight limitation(s) (2 points) PA1 b) no age limitation(s) (2 points) PA1 c) no time limitation (continuous measurability) (2 points) PA1 d) no place limitation(s) (portability of instrument) (2 points) PA1 e) simplicity of instrumentation (minimal proprietary item(s)) (2 points) PA1 a) price for testing for patient (2 points) PA1 b) price for instrumentation, e.g., buying price of instrument(s) (2 points) PA1 c) price for place, e.g., home, ambulatory or hospital, etc. (2 points) PA1 d) price for time, e.g., duration of test for doctor or technician(s) work (2 points) PA1 e) price for evaluation, e.g., assessment with graphical capability (2 points)
3. Economicor, considering:
4. Technical make-up, e.g., complication of procedure on the part of examiner or for the examinee (10 points)
5. No setting requirement, e.g., can be operated any place from field to hospital (10 points)
6. No patient risk from any point of procedure (10 points)
7. Availability in any health condition, e.g., post MI-Stress Testing or Cardiac Bypass-Operation and manageability of testing procedure in any body position, e.g., supine, standing or moving ( 10 points)
8. Instantaneous repeatability and/or continuous monitoring, e.g., under surgical procedure, postoperative care, etc. (10 points)
9. Real-time recognition of the patient's hemodynamic profile, e.g., instantaneous Computer Programming Capability (10 points)
10. Instantaneous availability and/or directions for therapeutic management (10 points).
Prior art cardiac output measurement methods use direct or indirect algorithms to make the displayed value as close to the actual cardiac output as possible.
The current clinical standard for invasive measurement of Cardiac Output, for example, implements the Stewart-Hamilton equation. A thermodilution computer calculates the different correction algorithms. Its inaccuracy, which can be .+-.20% from the actual cardiac output, is clinically acceptable and this method is used in spite of risk associated with this invasive method, its high cost, its intermittent capability of use and its other major inherent inconvenience, the possibility of infection.
A need exists, therefore, for the continuing development noninvasive methodologies to measure cardiac output. Presently only two types of noninvasive methods are available for continuous application: the Doppler ultrasonography and electrical bioimpedance methods.
The theoretical basis for the Doppler ultrasonography (continuous wave) method is the Doppler effect. Sound waves undergo a frequency shift when the distance between the generator and the receiver is changing. Doppler ultrasonography (Ultracom) is a reliable noninvasive procedure for one clinical setting and its accuracy is directly proportional to several technical assumptions. At best, the absolute accuracy of present continuous-wave Doppler ultrasonography systems is no better than .+-.45% of the actual cardiac output. The frequency of use is very limited and the systems are hospital-based. Furthermore, the results obtained by this method are user-dependent and the equipment requires a skilled operator.
The theoretical basis of the electrical bioimpedance method is the fact that the electrical conductivity of the thorax is proportional to the thoracic fluid content. Its changes are the result of volumetric and velocity variation of blood (the most electrically conductive substance in the body) in thoracic vessels. These measured variables, together with the volume of intrathoracic tissue (estimated by different algorithms of the computer from height, weight, and sex of the patient), are the basis for calculation of Stroke Volume and Cardiac Output.
BoMed Medical Manufacturing, Ltd., Irvine, Calif. markets its NCCOM3 instrument which uses an electrical bioimpedance methodology. BoMed's NCCOM3 has its clinically verified algorithms for cardiac output measurements within .+-.20% of the actual value in the majority of monitored patients. Compared to other methods, such as Doppler ultrasonography, this method has advantages; more user-friendliness, unlimited applicability and repeatability, increased accuracy and above all, it provides an estimate of volemic status and oxygen delivery.
However, these heretofore known methods have major shortcomings. With regard to continuous-wave Doppler ultrasonography, the maximum inherent inaccuracy and/or the absolute accuracy is not appropriate; the system is not patient or user-friendly, it is not a hands-off technique, there is inadequate accuracy at different flow levels, usability is limited (e.g. requires sterile environment) and it is not economical.
With regard to electrical bioimpedance measurements, there are the following shortcomings: Limited in-patient and user-friendly aspects; complicated technical make-up, e.g., physician's understanding of algorithms and their limitations; limited availability of the instrument (hospital setting); limited usability in some health conditions, concerning electrode-placements (proprietary items); inseparability of patient and instrument, e.g., the data obtained is inherently tied to the function of the instrument and one cannot use literature sources to reproduce previous evaluations; and lack of economy.
The present invention overcomes the problems which currently exist in the field of Cardiac Output measurement methods and is able to approach the ideal requirements noted above, up to about 94%, in contrast with the Doppler and bioimpedance techniques, which meet only about 50-70% of the ideal requirements. In particular, these methods are not able to meet the following requirements:
1. Simplicity of instrumentation and/or no proprietary item(s)
2. Economy--price of each instrument is prohibitively high, therefore, patient's expenses are increased
3. Technical make-up--complication of procedure
4. Setting-requirement--mostly hospital setting
5. Maneuverability of testing procedure.